Summary:
Mineral deposits at the Cerro Prieto mine site and surrounding areas are classified as low to intermediate sulfidation epithermal systems of silver and gold.
The local area, including the Project, consists of terrigenous shallow marine to deltaic clastic sediments equivalent to the Baucarit Formation (Nourse, 1995). Generally, this formation comprises polymictic conglomerates that are partly consolidated and cemented within a medium-grained matrix. These rocks can be observed on the north side of Cerro Prieto, where they form a discordant contact with the Cretaceous sediments and intrusions; however, they are otherwise rare in the Project area. Within the Project, these Tertiary sediments consist of thinly bedded fine to medium-grained arenites, exhibiting a general strike of 55° and a general dip of 30° to 45° to the northwest. Additionally, they are weakly metamorphosed and highly silicified.
Tonalite intrusions are present in the area, although they are not observed within the old mine. They range from red to gray, indicating alteration due to hydrothermal activity. The complete kaolinization of feldspar phenocrysts suggests that the alteration was intense. These intrusions are considered Middle or Upper Cretaceous age and are comparable to a granite/granodiorite batholith located east of the property.
Within the Project, Tertiary volcanic rocks include felsic volcanoclastics and andesitic to basaltic flows with minor rhyolite. They are disconformable located between the Tertiary arenites and polymictic conglomerates. In some areas, extensive rhyolite and dacite tuff beds have been deposited. Dyke rocks composed of andesite porphyry are present in the central part of the Project area. This is reported by Smith (1987); however, to the author's knowledge, no detailed surface map of the Project exists that shows the exact location of these.
Mineralization at Esperanzas
Mineralization at Esperanzas Deposit is confined to geological structures exhibiting approximately 345° Az in the primary structure and 313° Az in the secondary one. The main structure measures about 700 meters in length and features a semi-vertical dip, while the secondary structure's dip ranges from 65° SW in its northern section to around 50° SW in its southern portion.
In addition to these two prominent veins, the shear zone contains subsidiary zones of mineralization, including disseminations, veins, and stringers, which permit a wide range of low-grade mineralization.
Oroco's drilling campaign at Esperanzas Deposit confirmed that the mineralization consists of quartz veins, chalcedonic quartz veinlets, micro veinlets, amorphous silica, black calcite, and manganese lenses with argillic alteration.
Significant Mineralized Zones
Oroco has four potential exploration targets in progress. The following summarizes the geological characteristics of the Reyna, Cata-Coatí, and Cascabel exploration Deposits.
Geological Overview of the Reyna Deposit
Lithology
The Reyna Deposit consists of a volcaniclastic breccia containing angular to sub-rounded clasts within an andesitic ash matrix of Tertiary age. The rock exhibits colors ranging from ochre-brown to light brown and reddish gray, with darker brown shades in areas enriched with manganese oxides. The Baucarit Formation spans from the Oligocene to the Miocene epochs and is also exposed at the surface.
Structural Geology
• Multiple fault and vein systems characterize the project's structural framework;
• Primary Shear Zone and Veins: Exhibit a dominant strike of N75-80°W, dipping 70-80°SW;
• Puma-Type Vein System: This system, situated in the central-northeastern sector, comprises black calcite, chalcedonic quartz, and late-stage anhedral to subhedral white crystalline calcite. It trends N20-30°W, with dips varying from 50° to 70°SW;
• Central Normal Fault: Strikes N50-60°W with variable dips of 60-70° SW, featuring a gouge zone and apparent block displacement toward the southwest;
• NE-SW Fracture System: These fractures dip at 65-75° NW and define the boundaries of down-dropped blocks displaced to the southwest.
Mineralization
The Reyna Deposit is situated along the northern extension of the active Esperanza Deposit open pit. Exploration has identified gold mineralization within a shear zone trending NW75-85°SE, extending for at least 150 meters. This zone exhibits intense supergene alteration, high concentrations of iron oxides, and hematized points that indicate leached sulfides, primarily sphalerite (ZnS) and pyrite (FeS2). Moderate amounts of crustiform and chalcedonic quartz represent the most recent SiO2 event. Gold anomalies range from 0.25 to 25.27 g/t.
Geological Overview of the Cata-Coatí Deposits
Lithology
Shale-Sandstone Sequence:
• Distribution: Observed in outcrops across the Cata-Coati Deposits;
• Composition includes interbedded red, ochre, and greenish shale in massive to thick layers, featuring thin gray to reddish-gray sandstone beds;
• Characteristics: Fissile shale exhibits lamination with silt to medium grain sizes;
• Red coloration indicates an oxidizing depositional environment. Hematite and manganese oxides are present along bedding planes. Localized folding and foliation have been observed.
Limestone
• Occurrence: Mainly in low- to medium-thickness horizons above the shale-sandstone sequence;
• Type: Arenaceous/clastic limestone, gray with karst weathering;
• Composition: Well-cemented calcareous material;
• Alteration: Epigenetic silicification and recrystallization of quartz and calcite, with occasional quartz and calcite veining.
These lithological units correspond to the Late Jurassic Cucurpe Formation, dating back approximately 145 million years.
Sills and Dikes
• Composition: Rhyolitic and andesitic intrusive bodies;
• Rhyolitic sills are more abundant and thicker;
• Host rocks primarily intrude into the shale-sandstone unit along bedding planes.
Mineralization
Gold anomalies at the Coatí Deposit range from 0.2 to 6.65 grams per ton (g/t), while the Cata Zone exhibits anomalies between 0.2 and 18.83 g/t.
• Two mineralization stages have been identified, Amorphous Milky Quartz and Quartz Druses;
• Oxidation features include hematite, manganese, fine sulfides, and oxidized pyrite box-works;
• Orientation: NW-trending structures dipping 50°-75° NE;
• Gold Anomalies: Coatí - 0.2 to 6.65 g/t Au. Cata - 0.2 to 18.83 g/t Au.
Alterations
Alteration primarily occurs in mineralized structures, with some localized instances in other regions. Recognized types of hydrothermal alteration:
• Silicification is common in mineralized zones;
• Weak carbonatization;
• Weak propylitization;
• Oxidation, often associated with mineralization.
Geological Overview of the Cascabel Deposit
Structural Geometry
• The Cascabel structure is described as a breccia-type vein-fault system;
• It indicates a general strike of N30-45°W, with a dip of 65-70° to the northeast;
• A specific fault plane within the structure trends at N10° W and dips at 75° NE.
Host Lithology
The mineralized structure is located within shale-sandstone sequences (Js Pz-Ar).
The mineralization includes:
• Residual milky quartz is very porous;
• Crystalline quartz occurs in small drusy cavities;
• Manganese dioxide (MnO2);
• Oxides of hematite, goethite, and limonite;
• Hematized areas;
• Oxidized fine sulfides may form boxwork textures;
• Strong, pervasive silicification modifies the original texture and matrix of the host rocks;
• Brecciation and branching of milky quartz veins exhibit yellowish-green hues due to a moderate presence of chlorite and epidote.
Dimensions
The observable mineralized structure measures from 8 to 10 meters wide and extends from 40 to 50 meters from its outcrop to the boundary between Rancho Padres and Ejido Cucurpe.